<html>
  <head></head>
  <body>
    <div id="app">
      <div>targetMap</div>
    </div>
  </body>
  <script src="./reactivity.js"></script>
  <script>
    let product = { price: 5, quantity: 2 }
    let total = 0

    let effect = () => {
      total = product.price * product.quantity
    }

    track(product, 'quantity')
    effect()
  </script>
</html>
